I am having trouble meeting the CISPR11 class B emission limits. Can you advise on the best primary side supply voltage for lowest emissions; AN-0971 is not clear on this point. The secondary voltage is 3.3V and the load is about 30mA. At the moment the primary supply voltage is also 3.3V. Thank you

To choose which input voltage to use reduce emissions, AN-0971 says that emissions at 180MHz are proportional to PWM duty factor and independent of operating voltage. But the emissions at 360MHz are proportional to the load current and dependent on the operating voltage and will be better at 3.3V than at 5V from an EMI point of view.

The important technique to reduce emissions is the use of stitching capacitance as shown in AN-0971.

To choose which input voltage to use reduce emissions, AN-0971 says that emissions at 180MHz are proportional to PWM duty factor and independent of operating voltage. But the emissions at 360MHz are proportional to the load current and dependent on the operating voltage and will be better at 3.3V than at 5V from an EMI point of view.

The important technique to reduce emissions is the use of stitching capacitance as shown in AN-0971.

At a fixed power transfer, the PWM duty factor should be proportional to 1/Vin. If we have a 3.3V output with a fixed load, and we change the input from 3.3V to 5V, the PWM duty factor should go down proportionately. If the 180MHz emission peak must be reduced, then changing the input from 5V to 3.3V will reduce the 180MHz peak, but the 360MHz peak will not be affected.

Sorry for the confusion, but we don't have emissions data for 5V in and 3.3V out, just 3.3V in to 3.3V out, and 5V in to 5Vout. In figure 22 the 3.3V 180MHz peak emissions are a bit worse under the same load than at 5V. In Figure 23 at your 30mA load the 360MHz peak emissions are a bit better at 3.3V than at 5V. It may be that changing the input voltage may have a small effect, but we don't have the data to show this, you may want to try this out in your application at your load current.